The present invention pertains to the art of power transfer, particularly the transfer of electrical power between structures which are moving relative to one another. The invention finds particular application in conjunction with brush block assemblies for transferring high voltage electrical power and lower voltage electrical communication signals between the stationary and rotating gantry portions of CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with brush block assemblies for transferring power or communication signals in other environments.
Typically, a CT scanner has a center bore of 65 centimeters around which large diameter slip rings on the order of 70-75 centimeters in diameter are stationarily mounted. The x-ray tube is mounted in a rotating gantry portion mounted on bearings for rotation around the central cylinder. Arrays of brush assemblies were mounted to the rotating gantry portion to make electrical contact with the stationary slip rings in order to transfer high voltage electric power for operating the x-ray tube and electrical communication signals between the stationary rotating gantry portions.
One type of prior art brush blocks uses wire brushes. A wire brush consists of one or more wires arranged such that they function as a cantilever beam with its free end positioned against the conductive ring. Commonly, two brushes were used per ring to increase the current carrying capacity.
Wire brush assemblies are very delicate and easily damaged both before and during installation. Individual brush assemblies are not replaceable. Rather, the entire, relatively expensive brush block must be replaced. Wire brush assemblies use different slip ring arrangements than other brushes. Specifically, the slip ring is commonly a u-shaped groove whose inner surface is designed to contain and guide the wires during rotation in the normal direction. When the brush assemblies are changed to a different type of brushes, new slip rings must also be installed. Under rotation opposite to the normal direction, the wire brushes may hang up, buckle, and experience sudden catastrophic failure. High current overloads can vaporize the wires.
Another type of prior art brush is the composite brush. Composite brushes are commonly sold as a cartridge, i.e. a housing within which a block of conductive silver-graphite alloy material is slidably received. A cantilever, compression, constant force, or other spring is mounted within the housing to urge the block to extend, i.e. into contact with the slip ring.
One problem with the composite brushes is that the cartridges are relatively bulky. In many applications, such as CT scanner, the use of redundant cartridges, i.e. two brushes per ring, requires unacceptably large physical dimensions. When using only one brush per ring, the brush is apt to skip causing signal loss on communications signals or arcing on high voltage signals. In an effort to reduce the signal loss and arcing, relatively high spring pressures are used in the cartridge to bias the brushes hard against the slip rings. The high spring pressures decrease brush life due to a high frictional wear between the brush and the slip ring. The high friction quickly grinds the brush into conformity with a circular arc of the slip ring and wears it down leaving brush debris in the slip ring channels. Another problem with the cartridges is that the brush tends to hang up or stick. The cartridges tend to be relatively shallow compared to the length of the brush. Even small amounts of canting or tipping force from interaction with the slip ring can cause the brush to hang up or stick.
The present invention contemplates a new and improved brush block assemblies which overcomes the above referenced problems and others.